1. Field of the Invention
The present invention relates to a focus detecting apparatus for detecting a focus state of a taking lens of a camera or the like, with respect to an object to be photographed.
2. Description of Related Art
In a known focus detecting apparatus of a single lens reflex camera or the like, bundles of rays transmitted through different portions of an exit pupil of a taking lens are converged onto a pair of line sensors by an image reforming lens of a focus detecting optical system, so that the focus state of the taking lens can be detected in accordance with a relationship between outputs of the line sensors. The principle of the focus detection by the focus detecting apparatus is disclosed, for example, in U.S. Pat. No. 4,636,624.
A focus detecting apparatus having an off-axis detecting area (i.e., deviated from an optical axis) for detecting the focus state of a taking lens with respect to an object located at a position other than the center of an image plane is a known apparatus.
However, if there is a large deviation of the off-axis detecting area from the optical axis, vignetting of the focus detecting optical system pupil will occur. For example, vignetting will occur when the position or size of the exit pupil of the taking lens changes as a result of a lens change (i.e., when using an interchangeable lens) or during a zooming operation. In such cases, the area of the focus detecting optical system pupil, through which the bundle of rays is transmitted, is reduced. As a result, the types of interchangeable lenses that can be used in such an automatic focus detecting system are limited.
In particular, if the off-axis detecting areas are aligned in a radial direction of the taking lens, the bundle of rays corresponding to an upper portion of an image to be reformed may not reach the line sensors (i.e. in the case where an image height extends far from the optical axis).
FIG. 7 shows a known focus detecting apparatus having a focus detecting zone that is deviated from an optical axis Ax of a taking lens (not shown). A condenser lens 1 receives a bundle of rays which forms an image on a predetermined focal plane 2 (i.e. a plane which is equivalent to the film plane) from an exit pupil EP of the taking lens. Separator lenses 3 and 4 reform separate images of the bundles of rays transmitted through different portions of the exit pupil EP of the taking lens onto line sensors 5 and 6.
A known optical arrangement, shown in FIG. 7, has a taking lens with an exit pupil EP, which has been scaled down in size for illustrative purposes. In this optical arrangement, the condenser lens 1 has a symmetrical power distribution with respect to the optical axis 1x thereof. Accordingly, the bundles of rays defining the upper portions of the images to be formed on the line sensors 5 and 6 correspond to areas A and B outside the exit pupil EP, which results in vignetting (an eclipse) of an image due to the lack of rays corresponding to the upper portions of the images. In other words, an image which has a high image height can not reach the line sensors 5 and 6.